1. Field of the Invention
The present invention relates to tools used in electrical discharge machining. More particularly, the present invention relates to a tool for use in electrical discharge machining having interchangeable indicia marking electrodes.
2. Description of the Relevant Art
"Electrical discharge machining" refers to a method of removing metal from a workpiece. This type of machining is frequently used in tool and die applications as well as in the production of both prototype and production parts. Electrical discharge machining is particularly suited for the forming of odd shaped holes and slots which cannot be formed simply by known production methods. An electrode, or the cutting tool, is positioned adjacent the workpiece in a liquid environment. The relationship between the tool and the workpiece is such that a gap of between about 0.001-inches and 0.004-inches is formed therebetween. The tool and the workpiece are subjected to an electrical current. Because the tool provides one of the electrodes (the cathode) and the workpiece provides the other (the anode), the electrical current provides a string of quickly recurring electrical discharges in the gap between the workpiece and the tool. The rate may be varied from about 20,000 to hundreds of thousands of discharges each second.
The area forming the gap between the workpiece and the tool is itself immersed in a tank or continuous spray of a dielectric liquid, such as a hydrocarbon dielectric, through which the current flows to form the spark. In addition, the dielectric liquid cools and rinses the workpiece.
The process of electrical discharge machining is directed to the removal of metal from the workpiece by vaporizing and melting the target area of the workpiece. By altering the gap between the workpiece and the tool as well as the applied current and voltage, the amount of material being removed can be controlled. According to known techniques, the depth of etched areas can be between 0.0001 and 0.015, and the etched areas may be between a few thousandths of an inch wide to three inches wide.
The machine used today in electrical discharge machining includes a base for holding the workpiece, a tool perpendicularly moveable with respect to the base for adjusting the distance between the tool and the workpiece, and a tank for the liquid dielectric. The tool electrode is typically made of copper, brass, tungsten, graphite, or a zinc alloy. The shape of the electrode used today is typically a wafer shape, although continuous wire electrodes are being used with increasing frequency where precision cutting is required.
As may now be understood, the electrode is critical in determining the shape and depth of the hole being formed in the workpiece. Given the criticality of this feature, a variety of inventions directed at its improvement are known. For example, U.S. Pat. No. 3,719,579, issued on Mar. 6, 1973, to D. E. Cross et al. for a TOOL HOLDER discloses a device for holding a large number of closely spaced hollow electrodes. The device finds application in either electrical discharge or electrochemical machining where the simultaneous formation of several holes in a single surface is required.
U.S. Pat. No. 4,041,269, issued on Aug. 9, 1977, to Baker for a CARTRIDGE FOR AN ELECTRICAL DISCHARGE MACHINING APPARATUS discloses an electrode-holding cartridge for use in electrical discharge machining. The electrodes are fed to the nose guide of the machine. Once in their desired position, the electrodes are clamped in the cartridge.
Finally, U.S. Pat. No. 4,717,803, issued on Jul. 5, 1988, to Alexanderson for ELECTRODE HOLDER FOR ELECTRIC DISCHARGE MACHINES, discloses a holder for releasably holding an electrode. The electrode is released from the holder through rotation of a knurled knob.
While providing improvements in electrical discharge machining, the systems represented in these patents fail to provide utility for specific applications. Particularly, these systems fail to provide a practical method of etching letters, numbers or other characters (including logos) in a workpiece. While it is often desirable to etch such indicia in a workpiece, the only known method of doing this using electrical discharge machining is to provide a carbon electrode having a pantograph that is a cutting stylus. This system is slow and requires an extensive system for movement of either the workpiece or the carbon stylus along the X- and Y-axes as is necessary to "write" the character.
Accordingly, a versatile, easy to operate method of etching indicia on a workpiece is wanting.